1. Field of the Invention
The present invention relates generally to a pipeline safety program, namely a system of addressing pipeline anomalies prior to failure of pipeline integrity, and particularly to a pipeline inspection system integrating a novel serviceability acceptance criteria for pipeline anomalies, specifically wrinkles, with an improved method of correlating ultrasonic test data to actual anomaly characteristics.
2 Description of Related Art
Typically, a pipeline company will have a thorough pipeline safety program that will include a routine for the identification of pipeline defects and review of pipeline integrity. Such a plan should include, but should not be limited to: i) a review of previous internal inspection n report logs by a third party with demonstrated expertise in interpreting inspection report data; ii) excavation of sites identified by this review of the internal inspection report logs for visual examination of anomalies; iii) repairs as necessary; and iv) the use of internal inspection tool surveys and remedial action to the extent needed to address factors in the failure and verify the integrity of the pipeline.
A pipeline safety program can be only as effective as the interpretation of internal inspection reports. If the data recorded by an inspection pig is improperly classified, anomalies that would otherwise require repair may never be identified as serious enough to dig up and inspect. Nearly as problematic, on the other hand, is the great time and energy that may be spent digging up the pipeline searching for anomalies that do not, in fact, warrant inspection.
Proper classification of data recorded by the pig thus is essential for pipeline safety. For example, in one specific case as discussed more fully herein, a 1997 internal inspection of a length of pipeline using sophisticated Ultrasonic Testing (UT) technology identified an anomaly that was misclassified as a pipeline fitting, rather than the true defectxe2x80x94a wrinkle, which wrinkle later led to catastrophic failure of the pipeline. The vast majority of the deformation features examined in the field in this specific case were xe2x80x9cripplesxe2x80x9d or xe2x80x9cwrinklesxe2x80x9d that were evident in cold field bends and located on the intradose of the bend. These types of features are characterized by a xe2x80x9csinusoidalxe2x80x9d surface waveform with both an inside and outside displacement component. This particular event, and other recent pipeline construction experience (from the late 1980""s), has demonstrated that it is often difficult to produce field bends that have smooth contours without the presence of small discontinuities within the bend intrados. Such discontinuities have been referred to generally as xe2x80x9cbucklesxe2x80x9d, xe2x80x9cripplesxe2x80x9d, and xe2x80x9cwrinklesxe2x80x9d. Industry research into the structural integrity aspects of wrinkles or buckles in pipelines has been conducted for the past 25 years with the majority conducted since about 1990.
Since the late 1980""s, it has become increasing evident that the commonly used field bending xe2x80x9crule-of-thumbxe2x80x9dxe2x80x94that pipe could typically tolerate a maximum bend angle of 1.5 degrees per pipe diameterxe2x80x94can no longer be applied universally. It has been found that wrinkles in fact could be formed at smaller bend angles, for example, on the order of 0.75 to 1.0 degree per pipe diameter, and sometimes less.
Bending problems could often be traced to poor field bending practices including the improper setup of bending machines. However, other factors impacted the xe2x80x9cbendabilityxe2x80x9d of line pipe, including among others higher yield strengths, increased diameter/thickness ratios, and pipe steel properties, particularly the stress-strain behavior. It also had been found that the heat cycle associated with the application of fusion bonded epoxy coatings also promoted rippling at low bend angles due to alteration of the stress-strain behavior. In some cases, pipe produced to identical specification by different pipe manufacturers could not be bent to the same radius without wrinkling. Even pipe produced by the same manufacturer has exhibited bendability variations during pipeline construction.
Such pipe bending flaws were encountered worldwide, and led to industry research aimed at establishing engineering limits of acceptability for ripples in pipe bends. Thus, not only were there no serviceability acceptance criteria for pipeline wrinkles, but neither was there pipeline inspection data that could be used to develop such an acceptance criterion for wrinkles in pipe bends. Even while pipeline inspection tools became more and more sophisticated, there was no adequate method of correlating that data to represent the true characteristics of the anomaly, which type and severity of anomaly would be found upon repair digs.
Current US Code Requirements for Gas and Liquid Pipelines
For gas pipelines, 49 CFR Part 192 contains requirements for bends in Subpart G titled xe2x80x9cGeneral Construction Requirements for Transmission Lines and Mainsxe2x80x9d. With respect to bend contours, Paragraph 192.313 mandates that xe2x80x9ca bend must not impair the serviceability of the pipexe2x80x9d and that each bend must have a smooth contour without evidence of buckling, cracks, or other mechanical damage. Also, with some exceptions, longitudinal welds must be near as practical to the neutral axis of the bend. Paragraph 192.315 relates specifically to wrinkle bends in steel pipe. Wrinkle bends are not allowed in pipelines operating at 30% SMYS or higher, and below that, wrinkles must not contain xe2x80x9cany sharp kinksxe2x80x9d. Wrinkles must be separated by at least one pipe diameter and can""t have a deflection of more than 1.5 degrees each. The requirements in Chapter 4 of ANSI/ASME B31.8-1999, xe2x80x9cGas Transmission and Distribution Piping Systemsxe2x80x9d w are similar to those in 49 CFR 192. Paragraph 841.231 provides that bends xe2x80x9cshall be free from bucking, cracks, or other evidence of mechanical damage. Like 49 CFR 192, wrinkle bends only are permitted for operation at less than 30% SMYS and must not contain xe2x80x9csharp kinksxe2x80x9d.
With respect to liquids pipelines, 49 CFR Part 195 contains requirements for pipe bending in Subpart D, xe2x80x9cConstructionxe2x80x9d. Bending criteria provided in Paragraph 195.212 prohibit wrinkle bends while xe2x80x9ceach bend must have a smooth contour and be free from buckling, cracks or any other mechanical damage.xe2x80x9d Requirements in ANSI/ASME B31.4-1999, xe2x80x9cPipeline Transportation Systems for Liquid Hydrocarbons and Other Liquidsxe2x80x9d mandate that bends shall be free from buckling, cracks, and mechanical damage. (Paragraphs 404.2, 406.2, and 434.7).
The relevant US code sections therefore do not allow wrinkle bends in pipelines operating at more than 30% SMYS, and prohibit wrinkles anywhere in new pipeline construction.
Wrinkle Acceptance Criteria In Foreign Codes
Several foreign jurisdictions have extensively studied wrinkle problems, including the countries of Australia and Canada that have established acceptance criteria for anomalies like wrinkles. The first acceptance criteria for buckles in Australia was contained in a 1990 amendment to Australian Standard AS 2885-1987, wherein xe2x80x9ca buckle shall be deemed to be a defect where it does not blend smoothly with adjoining surfaces or its height is greater than 25% of the nominal thickness and the width of its base is less than eight times its heightxe2x80x9d. Pipeline field bending problems in Australia led to the research that resulted in changes reflected in the current revision of Australian Standard AS 2885.1-1997, xe2x80x9cPipelines-Gas and Liquid Petroleum Part 1: Design and Constructionxe2x80x9d. In this code, a buckle has been defined as xe2x80x9can unacceptable irregularity in the surface of a pipe caused by a compressive stressxe2x80x9d. The present code also differentiates between xe2x80x9cripples or bucklesxe2x80x9d formed during cold field bending, and those that may be formed as a result of other factors. In the latter case, the buckle height cannot be greater than 50% of the wall thickness, must blend smoothly with the adjacent pipe, and cannot reduce the internal diameter to less than the approved minimum value.
Section 6.6 of AS 2885.1 covers cold field bends. The bend acceptance limits in this Section include:
Unless approved by the operating authority on the basis of a specific test program, acceptance limits defined in the cold field bending procedure shall be as follows:
The height of any buckle shall not exceed 5% of the peak-to-peak length dimension in the Figures (or wave length).
Ovality shall not exceed 95%. (The minimum ID shall be 95% of the nominal value of the pipe being examined).
Surface strain shall not exceed the lessor of the strain tolerance of the coating being used or 10%.
Appendix J of this code is titled xe2x80x9cProcedure Qualification For Cold Field Bendsxe2x80x9d. At the present time, this appendix has also been designated as xe2x80x9cinformativexe2x80x9d which means that it is only for information and guidance. It provides background material needed to guide an operator through a comprehensive bend qualification procedure process.
Another code that contains criteria applicable to the acceptance of wrinkling in pipelines is Canadian Standards Association CSA Z662-1999, xe2x80x9cOil and Gas Pipeline Systemsxe2x80x9d. Pipeline design criteria are provided in Section 4 and Paragraph 4.3.1.1 states that xe2x80x9cthe designer shall be responsible for determining supplemental local stress design criteria for structural discontinuitiesxe2x80x9d. This includes the effects of denting and wrinkling on stress in pipelines.
In one respect the Australian and Canadian codes are similar. Although the Australian code does provide a buckle acceptance criteria, it implies that alternative criteria may be acceptable based on test data. The Canadian code places the responsibility for such analysis on the pipeline designer. In both cases, some level of wrinkle or buckle acceptance is provided for.
Relevant Industry Research
Industry testing has been conducted and includes the initial buckling phase and the post-buckling phase until failure occurred. Some information has been provided as to the conditions needed to promote different forms of buckling plus detailed results. A program was conducted in 1975 and reported in 1976 for Northern Engineering Services to support the design analyses and installation of a high pressure gas transmission pipeline in Canada. The intent was development of a structural design criteria to prevent wrinkling in pipe with and without external sleeve type crack arrestors. Loading due to pressure, temperature, and bending plus the stress state at the crack arrestor ends were considered.
Field bending problems resulting in ripples or wrinkles forming in the compression side led to concern regarding their impact on pipeline integrity. A project was funded by the Australian Pipeline Industry Association beginning in 1990 and completed 1993. This project was aimed at improving the understanding of the field cold bending process and development of acceptance criteria for ripples in bends. This activity resulted in a number of technical publications, and formed the basis for the current wrinkle acceptance criteria in AS 2885.1.
Due to similar concerns regarding field bending difficulties in the US, a project was launched by the Line Pipe Research Supervisory Committee of PRCI. This also included participation in the ongoing APIA project in Australia. Unlike the APIA project, the PRCI effort also included cyclic testing of pipe with ripples. These tests indicated that a large number of cycles (as compared to the number of cycles accumulated in service) would be required to cause failure in a ripple.
Thus, a review of related art indicates that while wrinkles have begun to be addressed in national codes in foreign jurisdictions, it remains apparent that globally the engineering aspects of pipeline wrinkles are little understood. Further, it appears unknown to correlate data gathered from a UT inspection pig to wrinkle deformation. While sophisticated pigging techniques are known, and representatively patented in U.S. Pat. Nos. 6,100,684, 5,864,232, 5,454,276, 5,115,196, 4,747,317, 4,430,613 and 4,072,894, just to identify a few, it still can be seen that a need yet exists for a pipeline inspection system comprising both a novel serviceability acceptance criteria for pipeline wrinkles, and an improved method of correlating ultrasonic test data to actual anomaly characteristics. It is to the provision of such a pipeline inspection system that the present invention is primarily directed.
Briefly described, in a preferred form, the present invention is an integrity verification program for product pipelines, which program comprises the successful integration of at least three subject areas: pipeline stress analysis, detailed assessment of UT data and disposition of excavated defects uncovered by the UT data. Prior to the present invention, it was unknown whether UT inspection tools could offer any detail regarding pipeline wrinkles.
The present invention arose in relationship with engineering work with a specific pipeline that suffered a leak as a result of a wrinkle that failed. The Piney Point Pipeline is a hot oil pipeline and is comprised of 51.5 miles of steel line pipe insulated with 1 to 2 inches of urethane foam and coated externally with an extruded polyethylene coating. The portion of the pipeline from Piney Point to Ryceville (30 miles) is comprised of 16-inch OD by 0.219-inch w.t. API 5L Grade X42 ERW line pipe. At Ryceville the pipeline is split into two branches, one serving the Morgantown generating facility and one serving the Chalk Point generating facility. Both branches are comprised of 12.75-inch OD by 0.203-inch w.t. API 5L Grade X42 ERW line pipe.
As with most buried pipelines, a hot oil pipeline is generally assumed to be fully restrained. That is, when pressurized with hot oil, it is thought to become stressed as the result of its inability to expand (or contract) due to a change in temperature and due to the xe2x80x9cPoissonxe2x80x9d effect of internal pressure. Full restraint means that no strain along the axis of the pipeline is permitted to occur. Because the axial stress that results from the restraint can be quite significant, movement of the pipeline may occur in areas where restraint is reduced or lost entirely. Reduction or loss of restraint can occur at bends in the pipeline, in areas of very weak soils, and in the vicinity of points where the pipeline comes above ground. Some movement is tolerable as long as the coating is not damaged by the movement and as long as the movement does not cause buckling of the pipeline.
The Piney Point Pipeline is used intermittently. When in use, No. 6 oil at a maximum temperature of 160xc2x0 F. is pumped, usually but not always, from Piney Point to one or the other of the two generating facilities. The maximum operating pressure level is 400 psig at the discharge of the pumps. Once the delivery of No. 6 oil has been completed, ambient temperature No. 2 oil is pumped back into the pipeline to flush the No. 6 oil out of the system into heated storage tanks. The temperature of the No. 2 oil is believed to never be below 50xc2x0 F. and maximum operating pressure for pumping No. 2 oil is also set at 400 psig.
The operation described above results in a cycle of longitudinal stress, but one for which the pipeline is presumably designed. However, a Apr. 7, 2000 release (xe2x80x9cSwanson Creekxe2x80x9d release) occurred at a buckle which, according to the National Transportation Safety Board""s metallurgist, ruptured as the result of progressive cracking in stages presumably from repeated cycles of operation. The cause of the buckle has not yet been fully established, but it is possible that its formation was facilitated by loss of restraint in the particular location of the release.
Following the April 7th release, it was ascertained that the presence of the subject buckle was evident on an internal inspection log in terms of an anomalous reading (loss of ultrasonic signal). The internal inspection tool in this case was run for the purpose of detecting corrosion-caused metal loss, and it has limited capability to quantitatively characterize a buckle. The appearance of the buckle as an xe2x80x9canomalyxe2x80x9d on the log when it was reassessed after the failure led to investigation for other like and similar anomalies at other locations. Though no anomaly exactly like the one representing the subject buckle was found, numerous smaller loss-of-signal anomalies were discovered. Upon excavation of representative samples, the anomalies turned out to be wrinkles in the pipe. Though none of the wrinkles was anywhere near as severe as the subject buckle, their existence suggests the need to assess their significance.
It appears that many of the other anomalies corresponded to the type of diamond-shaped wrinkles that can occur when a piece of pipe is subjected to excessively localized deflection during a cold field-bending procedure. It is speculated that because the pipe was cold bent with the urethane foam already on the pipe, the bending contractor was unable to notice that some of the bends were wrinkled. Therefore, whereas the wrinkled bends might have been rejected if noticed, a number of them were installed in the pipeline. The number of locations of such potential wrinkles is large, and it is desirable not to have to address each and every one in terms of remedial measures. It has been established over the past 10 years that minor wrinkles do not pose a significant threat to the integrity of a pipeline.
The analyses described below address the following topics.
Design of the pipeline as per ASME B331.4
Restraint, soil friction, soil passive resistance
Buckling resistance of straight, buried pipe
Propensity of elastically curved pipe to become wrinkled in service
Forces and displacements at bends
Cyclic life of wrinkled pipe.
Several digs were examined to prepare a comprehensive inspection data verification effort in order to provide a clear understanding of the UT tool capabilities. The excavations reliably establishes repeatable relationships between pipe surface deformation patterns (ripples/wrinkles) and UT image information. Considerable knowledge was gained as the result of these efforts that can allow pipeline assessments to be made confidently after inspection by a UT tool, rather than conducting a deformation tool inspection.
Specific field excavation sites were selected from the population of UT xe2x80x9cfeaturexe2x80x9d types for the purpose of providing a qualitative understanding of how the UT image type and degree of severity are related dimensionally to the physical shape/condition of the pipe surface. The field data/measurements obtained also allowed direct assessment relative to originally submitted accept/reject criteria for wrinkles.
The vast majority of the deformation features examined in the field were xe2x80x9cripplesxe2x80x9d or xe2x80x9cwrinklesxe2x80x9d that were evident in cold field bends and located on the intradose of the bend. These features are characterized by a xe2x80x9csinusoidalxe2x80x9d surface waveform with both an inside and outside displacement component. An acceptance/rejection criterion initially was specifically established for these inspections based on the Australian Code and approved by DOT. This original (industry-based) ripple/wrinkle rejection criteria for new pipe was a surface xe2x80x9cwavexe2x80x9d height greater than 1.5 t (t=wall thickness) and a wave xe2x80x9cAspect Ratioxe2x80x9d of less than 12 (aspect ratio=wave length/wave height).
From the detailed analysis of these excavations and the UT data, localized stress data was investigated. The localized stress analyses for various potential wrinkle geometries has led to the conclusion that wrinkles that are less than 180xc2x0 circumferential arc and have an aspect ratio greater than 7.5 are fit for continued service and need not be repair.
Extensive finite element analyses (FEA) of wrinkle geometries have identified three key characteristics for a wrinkle that control stress levels:
Circumferential Extent;
Wrinkle Aspect Ratio (wrinkle axial extent divided by peak to peak height); and
Wrinkle Profile.
The FEA analysis concludes that a wrinkle is unacceptable if its circumferential extent exceeds 180xc2x0 or if its aspect ratio (wavelength divided by height) is less than 7.5. Wrinkles with smaller aspect ratios and larger circumferential extents could accentuate with repeated cycling and produce cracks that could extend by a low cycle fatigue propagation mechanism during thermal cycles. The presently proposed acceptance criteria is to allow wrinkles with an aspect ratio of 7.5 or greater and a circumferential extent of 180xc2x0 or less. These criteria are shown by analysis to conservatively assure that wrinkles left in service are geometrically stable and have adequate fatigue lives, and are thus fit for continued service.
A detailed assessment of UT inspection data with respect to wrinkles was performed in an attempt to understand and characterize the types of UT signals associated with such features. The following can be concluded from this particular initiative:
UT inspection data provides adequate information to quantify the circumferential extent and wave form of wrinkle;
There is a good correlation between circumferential extent of wrinkles and aspect ratio for the features that have been investigated by excavations. Thus, the UT data has provided a sufficient basis for selecting and excavating the features that are a potential structural integrity concern; and
There is a very good correlation between the actual field circumferential extent measurements and the UT inspection data associated with the wrinkles excavated and assessed.
The present invention further comprises several types of repair methods, in addition to pipe replacements, including steel reinforcement sleeves (Type B), Composite Sleeves (Clock Spring and Armour Plate), and the PII Epoxy-filled sleeve repair (ESR). The PII ESR is the preferably repair method since most of the locations that contain wrinkles are in proximity to relatively minor bends. These minor bends provide difficulty for a conventional steel reinforcement sleeve installation because they prevent the good fit that is required for effective sleeve performance. Further, the composite sleeves would not provide adequate resistance to axial load introduced by the thermal cycles. Therefore, the best repair option, other than replacing the affected section of pipe, is the PII ESR.
The present invention further comprises an acceptance criteria for pipeline wrinkles. In order to determine the acceptability of local wrinkle deformations in pipe bends, stress analyses were performed using the ANSYS finite element program. A series of finite element models for varying local wrinkle deformation geometries were analyzed using both elastic and elastic-plastic material properties. The wrinkle geometry parameters that were varied include aspect ratio (wrinkle axial length divided by the height), circumferential extent, and maximum height. A model of the Swanson Creek failure wrinkle geometry was also analyzed. The results of these analyses show that:
Elastic peak stress and reversing plastic strain increase with decreasing wrinkle aspect ratio and increasing circumferential extent.
For a given aspect ratio, elastic peak stress and reversing plastic strain decrease with increasing wrinkle height.
The elastic peak stress and reversing plastic strain for the Swanson Creek failure wrinkle geometry are significantly higher than other wrinkle geometries measured during investigative digs.
The investigation of the pipe failure at Swanson Creek identified the failure mechanism as being crack initiation by fatigue with the final rupture occurring by a ductile tearing mechanism. Therefore, it is reasonable to base acceptance criteria for existing wrinkles in the Piney Point pipeline on the remaining fatigue life above the estimated 150 operation cycles experienced to date. Two methods were used to assess the remaining fatigue life for the various wrinkle geometries investigated:
Fatigue Life Based on Experimental Data
Experimental fatigue data showing reversals to failure versus reversing plastic strain in laboratory tests of steels similar to the pipeline""s API 5L Grade X42 steel were compared to the reversing plastic strain in the wrinkle calculated by finite element analysis.
Based on the experimental fatigue data, all wrinkles with aspect ratios of 7.5 or greater and circumferential extents of 180xc2x0 or less have reversing plastic strain values that would result in failure after approximately 2,500 cycles. Additionally, the wrinkle geometry for the Swanson Creek failure was calculated to have reversing plastic strain that would result in failure after approximately 200 cycles. These results demonstrate that the wrinkles that meet the acceptance criteria have large margins on their remaining fatigue lives.
ASME Boiler and Pressure Vessel Code Design Fatigue Life
Section III of the ASME Boiler and Pressure Vessel Code provides rules for determining the design fatigue life of a component based on the alternating stress intensity from an elastic analysis. Finite element analysis was used to calculate the maximum alternating elastic stress intensity for the cases examined.
The results of these calculations show that all wrinkles with aspect ratios of 7.5 or greater and circumferential extents of 180xc2x0 or less have design fatigue lives greater than the 150 cycles currently experienced, signifying that they are acceptable for immediate return to service. Additionally, the wrinkle geometry for the Swanson Creek failure was found to have a design fatigue life 40% lower than 150 cycles experienced.
The methods of fatigue life assessment detailed above show that:
Wrinkles with aspect ratios of 7.5 or greater and circumferential extents of 180xc2x0 or less are acceptable for immediate return to service.
Wrinkles with an aspect ratio of 7.5 and a circumferential extent of 180xc2x0 have expected fatigue lives ten times greater than the Swanson Creek failure wrinkle geometry, based on experimental fatigue data.
Thus, an object of the invention is to provide an improved pipeline inspection system. These and other objects, features, and advantages of the present invention will be more apparent upon reading the following specification in conjunction with the accompanying drawings.